Disposable syringes are known in which a cylindrical barrel formed of transparent plastics material receives a piston which is slidable within the barrel. A shaft, which may be of cruciform or other, e.g. circular, section extends from the piston to a plunger handle for enabling the piston to be displaced along the barrel in a first or proximal direction to cause injectable fluid or body fluid to be drawn into the barrel via an aperture at one end of the barrel, or in a second or distal direction to cause the fluid to be expelled out of the aperture or to be injected into a patient via a needle.
Syringes of this type are generally sold as disposable items and are intended to be used only once to negate the risk of transmission of diseases between patients. However, such syringes suffer from the drawback that it is difficult to prevent such syringes from being re-used, which re-use increases the risk of transmission of serious, life-threatening, conditions such as certain bacterial infections, viral hepatitis, and HIV.
Numerous designs have been proposed for syringes which are intended to negate or reduce the risk of the syringe being re-used. However, there are considerable challenges involed in designing a syringe which meets all desiderata, including, without limitation:
(a) allowing aspiration or flashback of blood to check for correct location of the needle in a patient;
(b) ability to deliver variable doses;
(c) smooth operation;
(d) simplicity of manufacture and use
(e) ability to inject diluent (e.g. sterile water) into a vial of powdered/lyophilised drug, and/or allow agitation of vial or syringe contents to assist powdered drug to go into solution.
EP0925083B1 discloses a single use syringe comprising a barrel with an internal annular groove at the proximal end and a further annular groove near the distal end but spaced from it. The plunger is formed with an integrally moulded barb-like flange adjacent the head, which flange bears resiliently against the barrel interior wall. The flange is able to move unrestricted in either proximal or distal direction along the majority of the barrel interior wall; however, the flange is only able to pass the annular grooves in the distal direction. The syringe is supplied with the plunger not fully depressed, so that the restrictor flange is on the proximal side of the more distal of the two grooves. Liquid may be drawn up into the syringe until the flange encounters the proximal groove, which prevents the plunger being withdrawn completely from the barrel. Liquid may be discharged freely from the syringe by depressing the plunger, the flange passing just beyond the more distal of the grooves when the plunger head is moved to the extreme distal end of the barrel. In this position, the plunger is now prevented from being withdrawn again because the flange will not pass the groove in the barrel. This design is elegant and simple but suffers from a number of drawbacks, not least the fact that there will inevitably be a slight jolt as the flange passes the more distal of the grooves when an injection is being given, and this jolt is likely to be felt by a patient. Furthermore, because of the unrestricted movement of the plunger between the two grooves, the syringe could potentially be used again and again provided the plunger is never fully depressed. After unpacking a syringe, it is normal to cycle the plunger over a short distance to check that the plunger is free to move and, if it is not, to free it: sometimes there can be a degree of adhesion between the plunger head and the barrel due to the length of time of storage, or due to the effects of gamma sterilisation. This is particularly the case with plunger heads which have had silicone lubricant applied to them. During this movement it would be relatively easy to lock the plunger of this syringe by moving the flange past the distal groove. This design is the only one of which the inventors are currently aware which properly can be used to aspirate a flash of blood for checking needle position in a patient prior to injecting a drug.
U.S. Pat. No. 5,000,737 discloses a syringe having a single piece metal barbed restrictor element located between the plunger shaft and the cylindrical interior syringe barrel wall. The element has barbs facing towards the plunger which prevent movement of the plunger distally with respect to the element, and barbs facing the barrel which prevent movement of the element proximally with respect to the barrel. The restrictor element is initially located near the proximal end of the barrel; thus initial proximal movement of the plunger to draw up liquid is permitted as the plunger can slide past the restrictor in this direction. Subsequent depression of the plunger to deliver liquid is permitted because the restrictor can move distally with respect to the barrel, and hence when the plunger is depressed it carries the element with it. Further movement is of course prevented. This design has many similarities with some of the embodiments set out below; however, it does not permit aspiration of a flash of blood nor repeated movement to assist in reconstitution of lyophilised drug.
US2003/0060759 discloses a design which has similarities to that of U.S. Pat. No. 5,000,737, but also some important differences. It, too, utilises a single piece metal barbed restrictor element mounted between the plunger shaft and the barrel interior wall, and it employes outwardly facing barbs to restrict the motion of the element with respect to the barrel. In this design, however, the plunger shaft has a stepped form with a shoulder part way along it. The restrictor element has a spring tang which acts against the barrel and forces it against the plunger shaft. The restrictor starts out at the proximal end of the shaft; withdrawal of the plunger past the restrictor is permitted until an enlarged diameter portion of the shaft, near the plunger head, comes into engagement with the restrictor. At the same time, the proximal end of the restrictor snaps behind the shoulder on, the plunger; thereby movement of the plunger in either direction with respect to the restrictor is prohibited. The plunger can be depressed, carrying the restrictor with it to the distal end of the barrel, and then the plunger is incapable of further movement. This design is simple and has been used in a commercial vaccination syringe product. It suffers from the disadvantage that it may be used repeatedly, provided the user does not draw up the plunger to the point where the restrictor snaps into place on the reduced diameter part of the plunger shaft. Furthermore, once the restrictor has locked into place, which is of course the intention, aspiration of a flash of blood is not possible.
U.S. Pat. No. 5,222,942 discloses a design based on a ratchet system. A collar is installed in an initial distal position between plunger shaft and barrel. The plunger shaft is formed with annular ratchet teeth, and corresponding teeth are formed on the collar. The ratchet does not permit the plunger to be moved proximally past the collar, so when the plunger is initially drawn back in order to draw up liquid, it carries the collar with it to the extreme proximal end of the barrel where a formation on the barrel prevents the collar and plunger from being withdrawn completely from the barrel. The ratchet is such that the plunger may then be depressed past the collar to dispense liquid, and then of course the syringe is disabled. This design does not allow for aspiration of a flash of blood.
PCT/GB2004/005357 ('357), which is unpublished at the date of filing the present patent application, discloses a number of designs which are intended to allow for taking a flash of blood and possibly to allow for reconstitution of powdered drug. The designs described in '357 are intended to be simple and inexpensive and to have a smooth action which will not give rise to uncomfortable jolts when a patient is receiving an injection. The application is assigned to the assignee of the present application.
The designs desribed in '357 work on the principle of providing for a small amount of “lost motion” between a restrictor element and the syringe plunger, which allows for unlimited repeated distal and proximal movement over a distance which is just sufficient for the purpose of obtaining a flash of blood (or optionally for reconstitution of powdered drug). The distance may be carfully chosen so that it is sufficiently small to prevent, hinder or discourage repeated use of the syringe to deliver small doses of drug.
As stated above, achievement of all desiderata for a non reusable syringe in a single design is difficult, and there is normally a trade off between the complexity of the design and the the number of these goals which is achieved by the design. One potential issue with the designs described in '357 is that they will allow for drawing up and delivery of a volume of drug which is smaller than the usable volume of the syringe, and then drawing up and delivery of a further dose or doses until the total volume of the syringe has been used. For example, a 5 ml syringe may be used to draw up 3 ml of liquid drug and then administer it. At this point the restrictor element has been moved a corresponding distance along the plunger or barrel, but there remains a distance corresponding to 2 ml of syringe volume over which the restrictor may move before the syringe is fully disabled; therefore a further drawing up and delivery of a 2 ml dose of medicament is possible before the syringe is fully disabled